Surgical articles of copolymers of glycolide and ε-caprolactone and methods of producing the same

ABSTRACT

Novel copolymers of ε-caprolactone and glycolide useful in making surgical articles and particularly surgical sutures having Young&#39;s modulus of less than 250,000 psi. New and improved polymerization methods for producing the novel ε-caprolactone and glycolide copolymers.

This is a division of application Ser. No. 432,176, filed Oct. 1, 1982,and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to synthetic surgical devices having improvedproperties made from copolymers of glycolide and ε-caprolactone and,more particularly, to oriented filaments and sutures prepared from suchpolymers and to methods of manufacturing such polymers.

2. Description of the Prior Art

Homopolymers and copolymers of lactide and glycolide are well known inthe preparation of synthetic absorbable sutures as disclosed, forexample, in U.S. Pat. Nos. 3,636,956; 2,703,316; 3,468,853; 3,865,869,and 4,137,921. Also, in U.S. Pat. No. 3,867,190, it is known to includecertain cylic comonomers with glycolide including ε-caprolactone. Infact, the use of cylic ester monomers in the formation of polyesters forthe fabrication of synthetic surgical articles is well known in the art.The conventional polymerization method of forming polymers of the cylicesters is through ring opening polymerization. In U.S. Pat. No.4,300,565, there is disclosed surgical articles fabricated fromsynthetic absorbable copolymers formed by copolymerizing glycolide witha cylic ester monomer in a specific manner. Hence, it should beappreciated that broadly copolymers of ε-caprolactone and other cyclicesters, such as lactide or glycolide, are well known and described inthe art as well as are various methods for their production.

The synthetic absorbable sutures have gained considerable acceptance inthe surgical field; however, the "handleability" or the compliance;i.e., flexibility and "limpness", has not always been consideredsatisfactory in monofilament configurations. It is believed thatmonofilament constructions are more suitable for surgical uses than themultifilament or braided configurations as they tend to produce lessinfection and trauma at the wound closure site. However, themonofilaments tend to be stiffer and harder to handle than the braidedconfiguration of the same diameter. Over the years, various polymercombinations have been tried in an attempt to obtain the desired verydelicate interplay between the properties of suture absorbability, invivo strength retention, initial knot strength, and high compliance orlow modulus. These desired properties, other than absorbability, areobtained in some suture materials; for example, in those described incommonly assigned copending patent applications Ser. No. 311,829 filedOct. 16, 1981, and Ser. No. 338,407 filed Jan. 8, 1982. The suturematerials described have the desired strengths, compliance andflexibility but are not absorbable and, hence, are limited in their use.To the best of our knowledge the only synthetic, absorbable sutureswhich in some instances may have the properties as described above arethose made from polydioxanone as described in U.S. Pat. No. 4,052,988.

It should be appreciated, that to design molecular chains needed for theproduction of highly compliant absorbable materials, an obvious route isto copolymerize suitable comonomers or mixtures of pre-polymers andmonomers following procedures similar to those used in the formation ofcompliant non-absorbable sutures. However, such is not the case forthose polymers are of the AA-BB non-absorbable type. Furthermore,copolymerizing comonomers of glycolide and ε-caprolactone following theteaching of U.S. Pat. No. 3,867,190 which describes the copolymerscontaining 15% or less of the ε-caprolactone moieties does not producecompliant materials. Copolymers containing less than 15% caprolactoneare random in nature and the monofilaments made therefrom display highmodulus and low compliance. It is known that copolymers containing lessthan 85% glycolide moieties with random microstructure do not generallyoffer good fiber forming polymers because of their improper level ofcrystallinity. Hence it would be expected that copolymers containingmore than 15% caprolactone sequences would have poor crystallinity andbe virtually amorphous and unsuitable for the production of strongmonfilament suture materials.

SUMMARY OF THE INVENTION

The present invention describes new copolymers containing specificweight percents of epsilon (ε)-caprolactone and specific weight percentsof glycolide or a mixture of glycolide and lactide. These new copolymersproduce synthetic absorbable surgical articles having new and novelproperties and produce filaments or suture materials having desirablestraight and knot tensile strengths, controllable absorbability,suitable in vivo strengths while unexpectedly displaying unique highcompliance characteristics and low modulus. In accordance with thepresent invention, the new copolymers have a tensile strength of atleast 30,000 psi and a Young's modulus of less than 350,000 psi. When infilament form, sutures made from our novel copolymers preferably have atensile strength of at least 50,000 psi and a Young's modulus of lessthan 250,000 psi. The novel copolymers of the present invention comprisefrom about 20 to 35 weight percent of ε-caprolactone and from 65 to 80weight percent of glycolide or mixtures of glycolide and lactide. Inpreferred embodiments of the present invention, when mixtures ofglycolide and lactide are used, the mixture should contain less than 20percent by weight of L(-)lactide. The new copolymers may be used asmolded or shaped articles or they may be fabricated into filaments andappropriate sutures by techniques well known in the art and may haveneedles attached to said suture as desired. The filaments may beannealed to produce materials having tensile strengths of at least50,000 psi while maintaining a Young's modulus of less than 250,000 psi.Our new copolymers may be designed to retain in vivo strengths of atleast 40 percent after 7 days while being completely absorbed in vivo inless than 150 days. In certain embodiments of the present invention thenovel copolymers have inherent viscosities of at least 0.8 dl/g asdetermined on a 0.1 g/dl solution in hexafluoroisopropanol (HFIP) at 25°C. In certain embodiments of the present invention, our novel copolymershave a crystallinity of at least 5% and preferably at least 10%.

Also in accordance with the present invention, our novel copolymers areproduced by polymerizing a mixture of glycolide and ε-caprolactone inthe presence of from about 0.004 to 0.02 weight percent of catalyst. Thecatalyst may be a metal salt or oxide, preferably a tin salt or oxideas, for example, stannous octoate, dibutyltin oxide and the like. Thepolymerization is carried out at a temperature of below 250° C. for aperiod of time sufficient to produce a conversion of the monomers topolymer of at least 80%. In other novel processes for producing thecopolymers of the present invention, a first step is used to produce alow molecular weight copolymer of ε-caprolactone and glycolide. In thefirst step, the copolymer should comprise at least 50% by weight ofε-caprolactone to obtain an ε-caprolactone rich pre-polymer. The firststep is carried out at a temperature of below 220° C. and is followed bya second step wherein additional glycolide is added to the pre-polymer.This additional mixture is polymerized at a temperature of above 120° C.for a period of time sufficient to produce a conversion of at least 80%.

Polymers produced by the methods described above may be readily extrudedand drawn as is well known in the art to produce oriented filamentarymaterial. The oriented filaments may be used with or without annealingto produce sutures. Needles may be attached to the oriented filaments toproduce needled sutures. The sutures with or without needles may besterilized by well known sterilization techniques to produce new andnovel sterile surgical sutures. The polymers may also be fabricated byother techniques such as injection molding and the like and thensterilized by techniques well known in the art to produce new and novelsterile synthetic devices.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following description and examples, all parts and percentages areby weight unless otherwise specified.

The method of the present invention comprises either a single stage or atwo-stage polymerization process. In the single stage polymerizationprocess, an essentially random copolymer of glycolide monomer withε-caprolactone is produced. The polymerization is carried out in aconventional manner using a polymerization reactor equipped with heatingand stirring means. The polymerization is carried out in the presence offrom about 0.004 to 0.02 weight percent of a metal salt or metal oxide,preferably dibutyltin oxide or stannous octoate. The polymerization isconducted with pure and dry reactants and under a dry and inertatmosphere at temperatures sufficient to maintain the reaction mixtureat a temperature close to the melting point of the polymer beingproduced. The amount of ε-caprolactone should be sufficient so that inthe final copolymer there will be from about 20 percent by weight to 35percent by weight of the ε-caprolactone moieties. The amount ofglycolide used should be sufficient so that in the final polymer thereis from about 65 weight percent to about 80 weight percent of theglycolide moieties. The polymerization should be conducted for a timesufficient to have a conversion of the monomers to copolymer of at least80 percent and preferably more than 90 percent.

The following example describes a preferred copolymer of the presentinvention as well as a preferred method for producing the copolymer.

EXAMPLE I

A flame dried 100 ml glass ampoule equipped with a Teflon-coatedmagnetic spinbar is charged with 14.27 grams (0.125 mole) ofε-caprolactone, 43.53 grams (0.375 moles) glycolide 0.0591 grams1,6-hexanediol and a catalytic amount of stannous octoate (0.25 ml of a0.033 molar solution in toluene). The pressure in the ampoule is reducedto evaporate the toluene. The ampoule is repeatedly purged and ventedwith dry nitrogen and the pressure adjusted with dry nitrogen to about3/4 of an atmosphere. The ampoule is sealed with a flame. The sealedampoule is immersed in a silicone oil bath preheated to 100° C. Thistemperature is maintained for 15 minutes, with stirring as long aspossible, and the temperature increased to 150° C. which is maintainedfor 15 minutes. The temperature is raised to 190° C. and thepolymerization continued for 18 hours at 190° C. The resultant copolymeris isolated, chilled, ground, and dried under vacuum at roomtemperature. Some unreacted monomer is removed by heating the groundcopolymer under vacuum at 110° C. for 16 hours. Approximately 95 percentconversion of monomers to copolymer is obtained. The resultant copolymercomprises 23 percent by weight of ε-caprolactone moieties and 77 percentby weight glycolide moieties. The inherent viscosity of the resultantcopolymer is 1.66 dl/g as measured using a 0.1 g/dl solution inhexaflourisopropanol (HFIP) at 25° C.

EXAMPLE II

For comparison purposes, Example 6 described in U.S. Pat. No. 3,867,190which describes a glycolide copolymer with 15 weight pecentε-caprolactone is carried out.

A flame dried 100 ml glass ampoule equipped with a Teflon-coatedmagnetic spinbar is charged under dry and oxygen free conditions with6.0 grams (0.053 mole) ε-caprolactone, 34.0 grams (0.293 mole) glycolideand 0.12 glram litharge. After repeated purging with nitrogen thepressure is adjusted with nitrogen to about 3/4 of an atmosphere and theampoule is flame sealed. The sealed ampoule is immersed in a siliconeoil bath and heated to 145° to 150° C. The ampoule is maintained in thistemperature range for 31 hours. The copolymer is isolated, ground anddried under vacuum at room temperature. Some unreacted monomer isremoved by heating the ground copolymer at reduced pressure at 110° C.for 16 hours. The conversion of monomers to copolymer is approximately97 percent. The resultant copolymer comprises 15 percent by weight ofε-caprolactone moieties and 85 percent by weight of glycolide moieties.The resultant copolymer is practically insoluble in HFIP.

Attempts to extrude and draw the copolymer to produce an orientedfilament are unsuccessful as the copolymer undergoes degradation at thetemperature required to obtain a uniform melt.

EXAMPLE III

An attempt to make a suitable filament forming copolymer in accordancewith the teachings of U.S. Pat. No. 3,867,190 using the amounts andtypes of catalyst in accordance with the methods of the presentinvention is conducted.

A flame dried 100 ml glass ampoule equipped with a Teflon-coatedmagnetic spinbar is charged under dry and oxygen-free conditions with6.0 grams (0.053 mole) ε-caprolactone, 34.0 grams (0.293 mole)glycolide, and 0.90 ml of a 0.33 molar stannous octoate in toluenesolution. The pressure in the ampoule is reduced to remove the toluene.After repeated purging and venting with nitrogen the pressure isadjusted with nitrogen to about 3/4 of an atmosphere and the ampouleflame sealed. The sealed ampoule is immersed in a silicone oil bath andheated to 145° to 150° C. This temperature range is maintained for 31hours. The copolymer is isolated, ground and dried under vacuum at roomtemperature. Some unreacted monomers are removed by heating the groundcopolymer at reduced pressure at 110° C. for 16 hours. Approximately 97%conversion of monomers to copolymer is obtained. The resultant copolymercomprises 15 percent by weight of ε-caprolactone moieties and 85 percentby weight of glycolide moieties. The resultant copolymer is practicallyinsoluble in HFIP. The resultant copolymer is not extrudable andorientable so as to produce filament satisfactory for producing sutures.On trying to extrude the resultant copolymer it undergoes degradation atthe temperature range necessary to maintain a uniform melt.

In preferred embodiments of the single step method for producing thecopolymers of the present invention, it is desired that about 22 percentto 32 percent by weight of the ε-caprolactone moiety be obtained in thefinal polymer.

As previously described, an alternate novel method for producing the newcopolymers of the present invention is to initially form a low molecularweight pre-polymer of ε-caprolactone and glycolide. This pre-polymer isrich in ε-caprolactone; that is, it comprises at least 50 weight percentof ε-caprolactone. The pre-polymer is produced at temperatures belowabout 220° C. Once the pre-polymer is formed, additional glycolide orglycolide/caprolactone or lactide mixture rich in glycolide is added tothe pre-polymer and the resultant mixture further polymerized attemperatures of from about 120° C. to 250° C. and preferably from about180° C. to 240° C. This two step polymerization is carried out to aconversion of at least 85 percent.

The following is a specific example of this alternate method forproducing the novel copolymers of the present invention.

EXAMPLE IV

A flame dried multineck glass reactor is charged under dry and oxygenfree conditions with 71.8 grams (0.629 mole) ε-caprolactone, 31.3 grams(0.27 mole) glycolide, 0.0882 gram glycolic acid and 0.43 ml. of a 0.33molar stannous octoate in toluene solution. The reactor is outfittedwith an adapter with a hose connection and a dry mechanical stirrer. Thepressure in the reactor is reduced and the toluene removed. The reactoris purged and vented with nitrogen which is maintained at a pressure ofone atmosphere for the remainder of the run. The reactor is immersed ina silicone oil bath and heated to 120° C. which is maintained for 10minutes. Over the course of 30 minutes the temperature is increased to200° C. which is maintained for 20 minutes. The bath is allowed to coolto 150° C., the stirrer is stopped and the reactor withdrawn from thebath. A small sample, about 0.2 grams, of the reaction mass is withdrawnunder a blanket of nitrogen. The sample has an inherent viscosity of0.51 dl/g. To the reactor is added 45.6 grams (0.399 mole)ε-caprolactone and 185.6 grams (1.599 moles) glycolide. The reactor isreintroduced into the silicone oil bath. The temperature drops to 120°C. which is maintained for 10 minutes while providing good stirring. Inthe course of 15 minutes the temperature is increased to 205° C. whichis maintained for 4 hours.

The copolymer is isolated, ground, and dried under vacuum at roomtemperature. Some unreacted monomers are removed by heating the groundcopolymer at reduced pressure at 100° C. to constant weight. Aconversion of monomers to copolymer of approximately 87% is obtained.The resultant copolymer comprises 26 percent by weight of ε-caprolactonemoieties and 74 percent by weight of glycolide moieties. The resultantcopolymer has an inherent viscosity of 1.53 dl/g as measured using a 0.1g/dl solution in HFIP at 25° C.

EXAMPLE V

The procedure of Example I is essentially followed as set out in thatexample except that the ampoule is charged with 17.1 grams (0.150 mole)ε-caprolactone, 40.6 grams (0.350 mole) glycolide, 0.1182 gram (0.001mole), 1,6-hexanediol, and 0.25 ml. of a 0.033 molar stannous octoate intoluene solution. The sealed ampoule is immersed in a silicone oil bathpreheated to 100° C. This temperature is maintained for 30 minutes withstirring as long as possible. In the course of 50 minutes thetemperature is raised to 190° C. which is maintained for 7 hours. Thepercent conversion of monomers to copolymer is approximately 90% and theresultant copolymer has an inherent viscosity of 1.24 dl/g as measuredusing a 0.1 g/dl solution in HFIP at 25° C. The resultant copolymercomprises 23 percent by weight of ε-caprolactone moieties.

EXAMPLE VI

The procedure of Example I is followed as set out in that example exceptthat the ampoule is charged with 14.3 grams (0.125 moles)ε-caprolactone, 43.5 grams (0.35 mole) glycolide, 0.0591 gram (0.0005mole) 1,6-hexanediol, and 0.51 ml. of an 0.033 molar stannous octoate intoluene. The sealed ampoule is immersed in a silicone oil bath preheatedto 100° C. That temperature is maintained for 15 minutes and thenincreased over the course of less than an hour to 195° C. which ismaintained for 2 hours. The copolymer is isolated, ground and driedunder vacuum at room temperature. Some unreacted monomer is removed byheating the ground copolymer at reduced pressure at 110° C. for 16hours. The conversion of monomers to copolymer is approximately 90%. Theresultant copolymer has an inherent viscosity of 1.62 dl/g measured at25° C. at a 0.1 g/dl concentration in HFIP. The resultant copolymercomprises 17 percent by weight of ε-caprolactone moieties.

EXAMPLE VII

The procedure as set forth in Example IV is followed as set forththerein except the reactor is charged with 22.8 grams (0.200 mole)ε-caprolactone, 10 grams (0.0862 mole) glycolide, 33.8 mg (0.286 mmole)1,6-hexanediol, and 0.216 ml. of a 0.33 molar stannous octoate intoluene solution. The charged reactor is immersed in a silicone oil bathand heated to 190° C. over the course of 35 minutes. Heating isdiscontinued and the reactor in the bath allowed to cool to 120° C. overa period of 30 minutes. While maintaining the temperature at 120° C. andunder a nitrogen blanket 6.5 grams (0.057 mole) ε-caprolactone and 59.7grams (0.514 mole) glycolide is added to the reactor. The reaction massis maintained at 120° C. for 40 minutes with good stirring. Over thecourse of 15 minutes the temperature is increased to 195° C. which ismaintained for 21/2 hours. The copolymer is isolated, ground and driedunder vacuum at room temperature. Some unreacted monomers are removed byheating the ground copolymer at reduced pressure at 85° C. for 16 hours.A conversion of monomer to polymer of greater than 90% is obtained. Theresultant copolymer has an inherent viscosity of 1.60 dl/g as measuredduring a 0.1 g/dl concentration in HFIP at 25° C. The resultantcopolymer comprises 26 percent by weight of ε-caprolactone moieties.

EXAMPLE VIII

The procedure as set forth in Example VII is carried out as set forththerein with the exception that the reactor is charged with 22.8 grams(0.200 mole) ε-caprolactone, 7.7 grams (0.066 mole) glycolide, 0.1182gram (0.001 mole) 1,6-hexanediol and 0.25 ml. of 0.033 molar stannousoctoate in toluene solution. The initial polymerization is carried outat 150° C. and the reactor then further charged with 27.1 grams (0.233mole) glycolide. The polymerization is continued for approximately 21/2hours at a temperature of from 190° C. to 205° C. A percent conversionof greater than 80% is attained. The resultant copolymer has an inherentviscosity of 1.00 dl/g as measured using a 0.1 g/dl solution in HFIP at25° C. The resultant copolymer contains 23 percent by weight ofε-caprolactone moieties.

EXAMPLE IX

The procedure as set forth in Example VIII is followed as set forththerein except that 17.12 grams of ε-caprolactone and 10.15 grams ofglycolide are used initially, 30.47 grams of glycolide are added priorto the second polymerization and the second polymerization is carriedout at 205° C. for 61/4 hours. A percent conversion of approximately 90%is attained. The resultant copolymer has a viscosity of 1.23 dl/g asmeasured using a 0.1 g/dl solution in HFIP at 25° C. The resultantcopolymer contains 22 percent by weight of ε-caprolactone moieties.

EXAMPLE X

A series of experiments is run at various ratios of ε-caprolactone andglycolide as shown in the following Table 1. A flame dried 100 ml glassampoule equipped with a Teflon-coated magnetic spinbar is charged withε-caprolactone and glycolide in the amounts shown in the following tableand 0.1182 gram 1,6-hexanediol and a catalytic amount of stannousoctoate (0.25 ml of a 0.033 molar solution in toluene). The pressure inthe ampoule is reduced to evaporate the toluene. After repeated purgingand venting with nitrogen the pressure is adjusted with nitrogen toabout 3/4 of an atmosphere and the ampoule is flame sealed. The reactoris immersed in a silicone oil bath preheated to 100° C. This temperatureis maintained for 15 minutes with stirring and then raised to 150° C.This temperature is maintained for 15 minutes and then raised to 190° C.which is maintained for 18 hours. This procedure is followed withexamples a through h, however, with example i, the temperature is raisedto 205° C. which is maintained for 2 hours. The bath is allowed to coolto 190° C. which is maintained for the final heating period; the coolingperiod and final heating period total 18 hours. The polymers from eachexample are isolated, chilled and ground. The percent conversion andinherent viscosity as measured using a 0.1 g/dl solution in HFIP at 25°C. for each copolymer are given in the following Table 1.

                  TABLE 1                                                         ______________________________________                                             Grams                               Final                                Ex-  (Moles)    Grams      %            Wt.                                   peri-                                                                              ε-capro-                                                                         (Moles)    Con-  Inherent                                                                             ε-capro-                      ment lactone    Glycolide  version                                                                             Viscosity                                                                            lactone                               ______________________________________                                        a     2.84 (0.025)                                                                            54.86 (0.47)                                                                             98    insoluble                                                                             4                                    b     5.70 (0.050)                                                                            52.53 (0.45)                                                                             98    insoluble                                                                             9                                    c     8.56 (0.075)                                                                            49.33 (0.425)                                                                            98    1.45   14                                    d    11.42 (0.100)                                                                            46.43 (0.4)                                                                              97    1.48   18                                    e    14.27 (0.125)                                                                            43.53 (0.375)                                                                            97    1.41   23                                    f    17.12 (0.150)                                                                            40.62 (0.349)                                                                            97    1.39   28                                    g    19.98 (0.175)                                                                            37.72 (0.324)                                                                            97    1.39   33                                    h    17.12 (0.150)                                                                            40.62 (0.349)                                                                            95    1.74   27                                    i    17.12 (0.150)                                                                            40.62 (0.349)                                                                            93    1.61   25                                    ______________________________________                                    

EXAMPLE XI

A flame dried 100 ml. glass ampoule equipped with a Teflon-coatedmagnetic spinbar is charged with 22.8 grams (0.200 moles)ε-caprolactone, 34.8 grams (0.300 moles) glycolide, 0.1182 grams (0.001mole) 1,6-hexanediol, and 0.25 ml of a 0.033 mole stannous octoate intoluene solution. The reactor is immersed in a silicone oil bathpreheated to 100° C. This temperature is maintained for 15 minutes withstirring. The temperature is increased to 150° C. and maintained for 30minutes and then increased to 190° C. which is maintained for 17 hours.The polymer is isolated, ground and dried under vacuum at roomtemperature. Some unreacted monomer is remove by heating the groundpolymer at reduced pressure at 110° C. for 16 hours. A conversion ofmonomer to polymer of better than 90% is obtained. The resultantcopolymer has an inherent viscosity of 1.39 dl/g in HFIP at 25° at aconcentration of 0.1 g/dl.

In this example, the resultant copolymer contains about 37% by weight ofε-caprolactone moieties and the resulting copolymer is practicallyamorphous. It is unsuitable for manufacturing dimensionally stableoriented filaments and surgical sutures.

EXAMPLE XII

A flame dried 100 ml ampoule equipped with a Teflon-coated magneticspinbar is charged with 11.41 g. of ε-caprolactone (0.4 mole), 0.0739 g.1,b-hexanediol (0.625 m.mole), and a catalytic amount of stannousoctoate (0.25 ml. of an 0.033 molar solution in toluene). The pressurein the ampoule is reduced to evaporate the toluene. The ampoule isrepeatedly purged and vented with dry nitrogen and the pressure adjustedwith dry nitrogen to about 3/4 atmosphere. The ampoule is sealed with aflame. The sealed ampoule is immersed in a silicone oil bath preheatedto 100° C. This temperature is maintained for 15 minutes, stirring whenpossible, and the temperature increased to 150° C. and maintained for 15minutes. The temperature is raised to 190° C. and the polymerizationcontinues for 18 hours at 190° C. The resultant terpolymer is isolated,chilled, ground, and dried under vacuum at room temperature. Someunreacted monomer is removed by heating the ground terpolymer undervacuum at 110° C. for 16 hours; a weight loss of 2.8 percent isexperienced. The inherent viscosity of the resultant terpolymer is 1.48dl/g. in. in 0.1 g/dl solution in hexafluoroisopropanol (HFIP) at 25° C.The new synthetic absorbable copolymers of the present invention may beconverted to oriented filament materials by techniques of extruding anddrawing well known in the art for producing filamentous materials. Thefilaments may be sterilized with or without attached needles to producesterile surgical sutures as is well-known in the art. A preferredtechnique for extruding and drawing the copolymers of the presentinvention is described in the following Example.

EXAMPLE XIII

The copolymer is melt spun in an Instron Rheometer at a temperature atleast 10° C. above the melting temperature of the copolymer. A 40 mildie with a L/D ratio of 24 is used. A sheer rate of 213 sec⁻¹ is usedfor the extrusion. The extrudate is taken up through ice water and woundon a spool. The wound fibers are stored at reduced pressure for 2 to 24hours. The monofilaments are oriented by drawing in one or two stages.The drawn filaments are heat set by heating at the desired temperatureunder constant strain with or without allowing for 5% relaxation.

The filamentary materials are usually annealed as is well-known in theart under conditions which improve suture properties. The filaments maybe annealed under tension at temperatures of from about 50° C. to 120°C. for periods of time of from 1 hour to 48 hours. In preferredembodiments we anneal our filament materials under tension attemperatures of from 60° C. to 110° C. and at times from 4 to 16 hours.

The filament materials are tested for various physical properties suchas knot tensile strength, straight tensile strength, elongation, andYoung's modulus. The copolymers also may be tested for inherentviscosity, melting temperature and percent crystallinity.

The following describes the various test methods used to determine theproperties of the filament materials and/or the copolymers.

The characteristic properties of the filaments of the present inventionare readily determined by conventional test procedures. The propertiesare determined using an Instron tensile tester under the followingconditions:

crosshead speed (XH): 2 in/min

Chart speed (S): 10 in/min

Sample length (GL): 2 in

Scale load (SL): 21 lbs/in.

Young's modulus is calculated from the slope of the stress-strain curveof the sample in the initial linear, elastic region as follows: ##EQU1##θ is the angle between the slope and the horizontal, XS is the initialcross-sectional area of the fiber (in²), SL is the scale load and XH,CS, and GL are as identified above.

The straight tensile strength is calculated by dividing the forcerequired to break (lbs) by the initial cross-sectional area of the fiber(in²). The elongation to break is read directly from the stress-straincurve of the sample allotting 10% per inch of horizontal displacement.

The knot tensile strength of a filament is determined in separateexperiments. The test article is tied into a surgeon's knot with oneturn of the filament around flexible tubing (1/4 inch inside diameterand 1/16 inch wall thickness). The surgeon's knot is a square knot inwhich the free end is first passed twice, instead of once, through theloop and pulled taut, then passed once through a second loop, and theends drawn taut so that a single knot is superimposed upon a compoundknot. The first knot is started with the left end over the right end andsufficient tension is exerted to tie the knot securely. The specimen isplaced in the Instron tensile tester with the knot approximately midwaybetween the clamps. The knot tensile strength is calculated by dividingthe force required to break (lbs) by the initial cross-sectional area ofthe fiber (in²).

The temperature profile of a copolymer is determined using aDifferential Scanning Calorimeter (DSC) by first heating the copolymerto its initial melting temperature (T_(m) initial) followed by rapidcooling the melted sample. The quenched coplymer is then reheated at arate of 20° C. per minute and the glass transition temperature (T_(g)),temperature of crystallization (T_(c)) and melting temperature (T_(m))observed. The crystallinity of the polymer as reported is measured byX-ray diffraction techniques as are well-known.

In all instances the inherent viscosity reported is measured at 25° C.at a concentration of 0.1 g/dl in hexafluorispropyl alcohol (HFIP).

The composition of the final copolymer is determine by NMR analysis.

The various copolymers produced in Examples I through XII are measuredfor one or more of the following properties: inherent viscosity, meltingtemperatures and percent crystallinity. The results of these tests aregiven in the following Table 2:

                  TABLE 2                                                         ______________________________________                                        Ex-  % ε-Capro-                                                                      Inherent T.sub.m                                               am-  lactone In                                                                              Viscosity                                                                              Ini- % Cry-                                           ple  Copolymer dl/g     tial T.sub.g                                                                           T.sub.c                                                                            T.sub.m                                                                            stallinity                         ______________________________________                                        I    23        1.66          12  75   165  33                                 II   15        Insoluble                                                                              225           210                                     III  15        Insoluble                                                                              221  22  115  203  26                                 IV   26        1.53     166  10  86   156  31                                 V    23        1.24                                                           VI   17        1.62     184  21  78   178  17                                 VII  26        1.60     185  14       188  35                                 VIII 23        1.00     222      110  213  23                                 IX   22        1.23                        33                                 Xa    5        Insoluble                                                      Xb   10        Insoluble                                                      Xc   15        1.45     225  20  84   201  40                                 Xd   20        1.48     221  18  98   200  34                                 Xe   25        1.41                                                           Xf   35        1.39                                                           Xg   35        1.39                                                           Xh   27        1.74     223  26  90   208  26                                 Xi   25        1.61                        21                                 XI   37        1.39                        practically                        XII  --        1.48     --   --  --   --   amorphous                          ______________________________________                                    

The copolymers of the present inention produced in accordance with thepreviously described Examples 1 through XII are converted to filamentmaterials where possible as previously described. In some instances thefilaments are annealed while in other instances they are not annealed.The resultant filament materials are measured for one or more of thefollowing properties: straight tensile strength, knot tensile strength,elongation, and Young's modulus.

The results of these tests are provided in the following Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Drawing Conditions         Annealing    Straight                                                                           Knot                                                                              Elong-                                                                            Young                    Example                                                                            Extrusion  1st Stage                                                                          2nd Stage                                                                           Conditions                                                                             Diam.                                                                             Tensile                                                                            Tensile                                                                           ation                                                                             Modulus                  No.  Temp °C.                                                                    Bath  (Draw Ratio/°C.)                                                                  Relax %                                                                            °C./hr                                                                     mils.                                                                             Kpsi Kpsi                                                                              %   Kpsi                     __________________________________________________________________________    I    240  Glycerine                                                                           5/52 1.2/74                                                                              5    80/6                                                                              7.8 88   54  30  38                       II   240  Glycerine                                                                           --   Not orientable to dimensionally stable                                                                --ber                                                                             --  --                       III  220  Glycerine                                                                           4/54 1.375/73                                                                            --   non-uniform fiber                                                                          --  --  --                       IV   180  Glycerine                                                                           4/52 1.51/71                                                                             0    110/5                                                                             7.8 89       43   79                      V    185  Glycerine                                                                           5/51 1.2/71                                                                              0    65/6                                                                              8.0 75   52  28  101                      VI   215  Glycerine                                                                           5/53 1.2/75                                                                              5    80/6                                                                              7.5 114  --  19  689                      VII  225  Glycerine                                                                           5/52 1.2/74                                                                              0     76/16                                                                            7.2 111  67  42  323                      VIII 180  Glycerine                                                                           4/61 1.5/60                                                                              0    65/6                                                                              8.1 59   43  25   94                      IX   185  Glycerine                                                                           5/49 1.2/72                                                                              5    80/6                                                                              7.5 85   65  25  156                      Xa   235  Glycerine                                                                           5/51 --    5    80/6                                                                              6.9 90   --  35  1131                     Xb   230  Glycerine                                                                           5/52 1.2/74                                                                              5    80/6                                                                              6.9 21   --  45  668                      Xc   235  Glycerine*                                                                          4/RT*                                                                              1.5/72                                                                              5    80/6                                                                              7.4 36   --  42  386                      Xd   240  Glycerine                                                                           5/54 1.2/70                                                                              5    80/6                                                                              7.3 56   67  17  301                      Xe   225  Glycerine                                                                           5/54 1.2/75                                                                              5    80/6                                                                              7.5 82   --  32  100                      Xf   220  Glycerine                                                                           5/51 1.2/73                                                                              5    80/6                                                                              7.5 55   --  32   88                      Xg   220  Glycerine                                                                           5/53 1.2/74                                                                              5    80/6                                                                              7.5 21   --  59   38                      Xh   240  Glycerine                                                                           5/54 1.2/74                                                                              5    80/6                                                                              7.1 62   50  40   48                      Xi   180  Glycerine                                                                           5/56 1.2/70                                                                              5    80/6                                                                              7.1 62   50  40   48                      XI   Not orientable                                                                           --   --    --   --  --  --   --  --  --                       XII  230  Glycerine                                                                           5/51 1.2/74                                                                              --   None                                                                              7.5 63   51  59   75                      __________________________________________________________________________     *1st stage at room temperature in air; second stage in glycerine.        

Fibers made from copolymers produced in accordance with some of theExamples previously described are annealed and sterilized and tested forabsorption characteristics. The percent breaking strength retentionafter various lengths of time is determined.

The breaking strength of a sample is determined by implanting twostrands of a sample in the dorsal subcutis of each of eight (8)Long-Evans rats. Thus, 16 strands of each sample are implantedcorresponding to the two implantation periods; eight examples of eachsample for each of the periods. The periods of in vivo residence are 7and 14 days. The ratio of the mean value (of 8 determinations) of thebreaking strength (determined with an Instron Tensile tester inaccordance with standard testing procedure) at each period to the meanvalue (of 8 determinations) obtained for the sample prior toimplantation constitutes its breaking strength for that period.

Table 4 provides the results of the breaking strength retention for theexamples as indicated.

                  TABLE 4                                                         ______________________________________                                                                    % Breaking Strength                               Example Processing Conditions                                                                             Retension At                                      No.     Annealing  Sterilization                                                                              7 days                                                                              14 days                                 ______________________________________                                        IV      5 hr./110° C.                                                                     Ethylene Oxide                                                                             44    11                                      VII     16 hr./76° C.                                                                     Cobalt 60    62    37                                      I       6 hr./80° C.                                                                      Cobalt 60    54    13                                      Xh      6 hr./80° C.                                                                      Cobalt 60    52    12                                      Xi      6 hr./80° C.                                                                      Cobalt 60    49    11                                      Xe      6 hr./80° C.                                                                      Cobalt 60    58    24                                      Xf      6 hr./80° C.                                                                      Cobalt 60    61    22                                      ______________________________________                                    

The filaments of the present invention may be used as mono-filament ormultifilament sutures and may be woven, braided, or knitted. Thepolymers of the present invention are also useful in the manufacture ofcast films and other solid surgical aids as are well known in the art.

Many different embodiments of this invention will be apparent to thoseskilled in the art and may be made without departing from the spirit andscope thereof. It is understood that this invention is not limited tothe specific embodiments thereof except as defined in the appendedclaims.

What is claimed is:
 1. A process for producing a copolymer of glycolideand ε-caprolactone comprising:forming a low molecular weight pre-polymerof ε-caprolactone and glycolide, said pre-polymer comprising more than50 weight percent of ε-caprolactone, and being produced at a temperatureof below 220° C., and adding additional glycolide to said pre-polymerand polymerizing said mixture containing the additional glycolide at atemperature above 140° C. for a period of time sufficient to produce aconversion to copolymer of at least 80 percent and copolymer having acrystallinity of at least 5 percent.
 2. A process according to claim 1in which the pre-polymer has at least 60 weight percent ε-caprolactone.3. A process according to claim 1 wherein the low molecular weightpre-polymer is produced using metal salt or metal oxide.
 4. A processaccording to claim 2 wherein from about 0.004 to 0.02 weight percent ofcatalyst is used.
 5. A process according to claim 4 wherein the catalystis stannous octoate.
 6. A process according to claim 5 wherein aconversion to copolymer of at least 90 percent is obtained.